Chapter III Model of the pre-existing solar thermal energy system at UKZN
3.2 Model of the pre-existing solar thermal energy system at UKZN
3.2.1 System description
The plant under consideration is a small solar thermal system, intended for home use, such as water warming and food preparation or other home scale applications in rural communities. This was a project aimed at improving the UKZN former air based system addressed by (Heetkamp, 2002). Following is a summary description of the system components, which will give a basic understanding of the overall working principles of the system and help in understanding the system behaviour. This would assist in the design of a MCU based monitoring and control system.
3.2.1.1 The energy capture subsystem
This is composed of the following subcomponents:
1. A paraboloidal half dish concentrating collector with an aperture diameter of about 2.4m. The reflecting surface (built by another team member) is composed of trapezoidal tiles. Figure 34 shows two photographs of the concentrator.
2. A heat receiver/exchanger: It is a spiral coiled steel pipe of 10mm external diameter (and 8mm internal), forming a 20cm circumference of receiving surface. The receiver’s steel pipe is part of the overall circuit followed by the heat exchanging fluid in the charging cycle, according to what is depicted in the Figure 33.
3. A supporting and tracking assembly, which is a dual axis polar mount tracking system, compatible with the one described in the section 2.1.3.3.1.4 “Polar (equatorial) tracking”. One DC motor fitted on each axis, being the actuators that provide the angular movement.
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Heat exchanging fluid
Half dish collector
Charging pump Discharging pump Sol
Heat receiver
Thermal energy Storage (TES)
Heat utilization volumetric expansion beaker
It should be mentioned that the paraboloidal half dish and the receiver were not actually pre-existing. They were built, in parallel with the present work by another team member.
3.2.1.2 The thermal energy storage (TES) subsystem
It is made of a cylindrical steel can (of approximately 20 litre), filled with pebble and the heat exchanging fluid. The TES inlet is at the top and the outlet at the bottom. At the bottom there is also an auxiliary oil inlet/outlet to a transparent and graduated small (~2.5l) beaker for adding oil to the storage or for volumetric expansion compensation that may arise from the heat exchanging process. As with the receiver, the TES is part of the heat exchanging fluid charging circuit, according to what is represented in the Figure 33.
3.2.1.3 Energy utilization subsystem.
The heat utilization is a spiral coiled copper pipe that allows a normal sized pot to be placed over it (this arrangement can however be replaced by any other suitable home application like water heating). The heat transfer fluid (the same as of the charging circuit) circulates between the TES and this pot, forced by the discharging pump at a speed that must be governed by heat transfer laws in accordance to the cooking process needs.
3.2.1.4 Fluid / heat transport
Following is a description of the parts that perform the fluid and heat transport among the different subsystems. The fluid/heat transport system is composed of:
• A heat transfer fluid: Calfo AF (further addressed below),
• The fluid charging circuit (left loop in Figure 33). This links the receiver to the TES,
• The fluid discharging circuit (right loop in Figure 33). This links the TES to the
Figure 34 - Photos showing 2 details of the colector and tracking assembly at roof Physics, UKZN
heat utilization subsystem,
• A charging circuit pump, fitted between the TES outlet and the receiver’s inlet. This is the most suitable position, considering that, it lies on the cold pipe of the charging circuit (this remains true until pump’s critical temperature is reached in the charging process), and
• A discharging circuit pump, fitted between the TES outlet and the receiver’s inlet. It lies also on the colder side of the discharging circuit.
These pumps are cheap and of low operating temperature. No datasheet or any other information about their operating settings were found. They are presumably of 100ºC/150ºC operating temperature. Each pump is driven by a DC motor aimed at producing flow rates at such levels required by control laws, according to the particular heat transfer needs (at charging and/or discharging processes). On the other hand, it is worth noting that the pumps and their motor drives are old and needing replacement.
3.2.1.5 The Heat Transfer Fluid
It is important to describe briefly the characteristics of the heat transfer fluid to understand the manner in which it can affect the TES charging and discharging process or the overall system dynamics.
The fluid used, Calflo AF oil, has the summary properties listed below. Details can also be found in its datasheet in appendix D, and (Petro-Canada, 2006). The datasheet includes plots that relate temperature to heat capacity, density, thermal conductivity and viscosity. The main properties of the Calflo AF heat transfer fluid are:
i. Maximum operating temperature of 316ºC: not that good for the intended applications (ex: baking is at about 250ºC );
ii. Thermal conductivity of 0.142 to 0.127 W/(m K): low and variable;
iii. Specific heat capacity of 1.89 to 2.88 KJ/(Kg K): variable;
iv. Viscosity of 32.1 to 0.7 cSt: high at low temperatures and variable;
v. Density of 0.88 to 0.68 Kg/L: variable.
On the other hand, as can be seen (from the plots), density, heat capacity and thermal conductivity vary linearly with temperature over the operating range, while viscosity has a non linear variation, being high at low temperatures and vice- versa.
This variability of fluid characteristics introduces additional complexity to the system dynamic model. In addition, some of the properties (or their conjunction) may compromise system performance in a moderate to severe fashion. In effect, as also addressed by Løvseth (Løvseth, 2008), for efficient heat transfer with this oil, a turbulent flow (implies Reynolds number Re > 4000) is mandatory. It is difficult to achieve a turbulent flow below 250ºC, due to the Calflo AF’s high viscosity at low temperatures (because Re is in inverse proportion to the viscosity).
The need for achieving such turbulent flows will require high pumping power at low
temperatures, to counteract the high viscosity and high density that is characteristic of the oil at low temperatures. In this way, the already built pumping system may or (most probably) may not be able to perform up to such requirements. This may prevent the control system from performing the expected work to a satisfactory level.
Also, the maximum temperature of 316ºC, is not only a performance constraint but also a safety constraint as addressed below (section 3.2.1.7).
3.2.1.6 Data acquisition and control subsystem
The system that was developed previously by Mawire (Mawire, A., 2005) and also by Robert van den Heetkamp (2002) included a data acquisition and control subsystem composed of:
a) 2 HP/Agilent 34970A data loggers, that performed data readings of relevant plant’s outputs and DAC generated actuating outputs to the plant;
b) 2 Desktop Windows PCs running data logger interfacing software, both for the data input and control data output.
• An electrical hot plate simulating a solar heat collector.
All these components along with the heat storage and the dish collector, formed part of a complete system in which heat was supplied by either the dish collector (Heetkamp, 2002) or the electrical hot plate (Mawire, 2005).
However, the system developed by Heetkamp and Mawire was mostly dismantled. In particular, the data acquisition and control system was completely dismantled and attempts to rebuild it were fruitless.
The present work therefore, attempts rebuilding the data acquisition and control system, using an embedded microcontroller, together with a newly built half dish solar collector and the existing rock bed with its thermocouple sensors.
3.2.1.7 Safety considerations
Some precautions have to be taken to insure that the system is operated inside safety conditions. The intention here is to underline the temperature related safety considerations, where the control system can play a role. Each one of the system components described above, can only withstand a limited value of temperature:
(a)The heat transfer fluid should not be heated above 316ºC or its properties cannot be guaranteed thereafter. It is also important to observe that besides losing the normal operating properties there is also a risk of auto ignition (from 343ºC).
(b)The charging pump: It is located at the cold side of the charging circuit.
Nevertheless, it is important to insure that the fluid outlet temperature from the TES is not allowed to exceed the pump’s maximum operating
temperature (about 100/150ºC), otherwise the pump may be damaged.
This has the negative implication that the TES cannot be fully charged to temperatures above that mentioned above.
(c)Also the discharging outlet temperature from the utilization system should not be allowed to exceed the discharging pump’s maximum operating temperature.
(d)The receiver’s steel pipes: To insure that the melting temperature of steel (about 1370 °C) will not be attained, a stagnation (and higher) temperature should not be allowed to develop in the receiver. Such levels of temperatures can develop at the receiver if it is focused to the sun and the fluid flow is left null (pumping speed is zero).
(e)The receiver’s surface should not be allowed to exceed the maximum operating temperature of the absorbing material (about 3000C);
(f) The conducting pipes: There are flexible conducting pipes at the inlet and outlet of the receiver, made of ptfe (Teflon), with a maximum safe operating temperature of about 260ºC. So, it is important to insure that the fluid temperature is maintained below 260ºC.
Other oil conducting pipes are made of either steel or copper (about 1085°C melting point). So, if the previous and most of the other constraints are met, these steel and copper pipes are safe.
All these safety operating conditions, and others not mentioned, must be taken into consideration for a safe operation.
It is essential that the monitoring and control system be given that knowledge base and made capable of performing the required preventive actions or taking corrective actions whenever a critical condition is detected.
3.2.2 Summary of physical working principles of the thermal energy system